... They do fine calibration of the IMU on a regular basis - (normally cited as a 'QFE' I believe) - and I'm sure one would have occurred very close to the final location.

Thanks for that tip. I am currently trying to identify the latest fine calibration of the IMU, in order to determine when the last, most accurate, attitude was known. Apparently, it is actually called a QFA (get Quick Fine Attitude). The latest one I am sure of was on sol 2148. There might have been one done on sol 2163, but I am not sure. I am still a newbie with regard to digging through the Analyst's Notebook.

I grabbed a new quaternion from sol 2149 to see what Spirit's orientation looked like on the sol after the QFA. The results are below. Although the tilts are almost the same as those James posted earlier for the last known location(~50 sols later), the heading is about 25 degrees closer to north. I wonder, is there another Fine Attitude adjustment that is not "Quick" that I should be looking for?

Looking at these angles makes me wonder what the real pointing accuracy of a MER is. Or "was", for Spirit.

I mean, a figure down in the 10-15 of a degree range equates to an angle of around six inches at one light year. Almost-sort-of handy when doing the maths on multiple quaternion rotations (and then rounding down) but essentially meaningless in any real sense.

- The IMU is used whenever there is mobility. It tracks changes in attitude, by which I mean the vector or quaternion describing the rover's orientation (not just one element of said vector). The QFA or GFA (get fine attitude, which takes longer and is not quick) is run when the inertial measurement needs updating. The Sun measurements are critical to this.

- The Sun imaging in the QFA/GFA is not just a single image. It is a set. Position of the Sun in one image gives you two pieces of information (direction of the Sun, for instance in rover frame azimuth and elevation). Imaging over time gives you two more (d_az/d_t, d_el_dt). They are not fully orthognal, but are enough to populate roll,pitch,yaw or a quaternion (the quaternion is 4 numbers, but they quadratically add to 1, so only 3 degrees of freedom). A long IMU integration ensures that the attitude is over-determined. The orientation of the Sun in the frame is not determinable, except from infeerence based on motion.

- The frames of reference are the rover body frame and local level (i.e., areoid). I doubt the difference between "orthoganal to gravity" and "orthoganal to the radius vector" add up to much at either MER site.

- Time is actually part of the error budget, and affects the derivation from Solar images but not IMU. You couldn't refine longitude with solar imaging, since time knowledge can drift by up to 30 sec between corrections, and there are PMA-related uncertainties on top of that.

- The attitude is updated every time the IMU is used. A QFA/GFA updates the site. A drive does not end with the same attitude as it starts with. Those changes are tracked with the IMU, reported in the images, and used to point the HGA at Earth. This happens many times per QFA.

- The last quaternion I see, sol 2209, is [0.885949, -0.169903, 0.0031267, -0.431529] (Doug's). For that I get a tilt of 19.6 deg WSW, yaw=309 (51 W of N) pitch=-8.1 (nose down), roll=17.7 (+Y/right-hand panel up). So that's pretty close to what James got, no surprise there.

Mark: Thanks for taking the time to post those informative comments. You answered many of the questions that have come up in this discussion. I am especially pleased to get your verification on the orientation angles.

Now that we know that we are converting the quaternion data correctly, the only question remaining in my mind is how accurate is the final orientation. I did a little more digging around in the Analyst's Notebook and discovered that a "Get Quick Fine Attitude" was performed on sol 2164 and that there were only two subsequent drive attempts (one on sol 2165 and the last on sol 2169). These drives were attempts to improve the rover's northerly tilt for the coming winter. The mission Manager's reports indicate little improvement was achieved.

I am going to conclude that the final orientation is accurate, since there were only two attempts to move the rover after the final QFA.

I also found this nifty little map attached to the documentation from those sols showing Spirit pointing to the heading we expected.

Yes, thank you, Deimos, for taking the time for that informative reply! I've always been interested in how the MER INS works at this level of detail; again, I'm used to aircraft & therefore high speeds-over-ground with alignment referenced to a geophysical frame.

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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.

Now that we have Spirit's final orientation defined I can post the model for others to use in Google Mars. The model has been saved in the correct orientation, so all you'll need to do is drag it to Eduardo's final location in Google Mars. As long as you don't grab the "rotation" handle while you are adjusting it's position, it should appear in exactly the correct orientation.

I got this model originally as a Google Sketchup model. However, the original model was not correctly scaled. I scaled it to the correct size in Sketchup, oriented it to the angles provided by Deimos, adjusted it's vertical position so that the left side wheels would appear partially buried, and then finally exported it as a Collada model that can be easily imported into Google Earth/Mars.

I suspect there may be a general interest in the model, but I thought I would post it here first to see if it worked properly for people and didn't have any bugs I haven't spotted. If no one identifies any problems, I'll post it in the main Spirit thread.Final_Spirit_scaled_oriented3v7.zip ( 296.46K )
Number of downloads: 205

Once you have the model, it is easy to create your own custom views of Spirit in her surroundings.

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